Thermionic vacuum tube



May 16, 1933.

Fig. l.

H. M. FREEMAN ET AL THERMIONIC VACUUM TUBE- Filed Jan. 8, 1923WITNESSES:

@,M /ffmffeff 2 sheets-sheet 1 Wallace G.

INVENTORS Wade 87 Hulcsr M. Freeman.

' ATTRNEY My 16, 1933. H. M. FREEMAN n A1. 1,9Q9051 THERMIONIC VACUUMTUBE wlTNEsSEs': lNvENToRs Wallace G. Wade. 87

Huberi- M. Freeman.

BY W 'ATTORNEY Patented May-16,1933

i UNITED STATES PATENT or-Fl'cE HUBERT M. FREEMAN, F EAST PITTSBURGH,AND WALLACE G. WADE, OF PITTSBURGH,

PENNSYLVANIA,- ASSIGNORS TO WESTINGHOUSE ELECTRIC AND- MANUFACTUR- INGCOMPANY',y A CORPORATION 0F PENNSYLVANIA A THEBIIONIC VACUUM TUBEApplication led January 8, 1923. Serial No. 611,263.

Our invention relates to space-current devices and more especially tothe cathode structure of such devices.

The principal object of our invention is to provide a device of thecharacter described which may be employed for detecting, amplit fying orrectifying alternating currents and which embodies a cathode structureadapted methods of manufacture and which shall embody parts capable ofmanufacture in existing automatic machinery with minimum expenditures oftime and of money.

Heretofore', it has noty been practical to employ alternating currentsfor the excitation of the cathode or filament of a receiving oramplifying tube for the reason that such currents introduce variationsin the plate current of the tube. Such variations are thought to be dueto the following causes:

1. The variations in the intensity of the magnetic field established bythe alternating currents traversing the filament, thereby resulting in avariable deflection of the electron stream emanating from the filament;

2. The variations in the` electric field around the filament which arecaused -by the reversals in the potential-distribution along thefilament;

3. The variations in the emissivity which are caused by the alternateheating and cooling of the filament.

We have'found that the desirable results outlined hereinabove may beobtainedby applying a cathode construction having an operating cathodesurface which has no fall of potential along its surface, that is, aso-l called equipotential surface. Such cathode surface may be renderedthermionically activein a number of different Ways, as byl sub- ]ect1ngthe same to heat or to an .electron bombardment. In one form ofembodiment of ourlnvention, we provide a cathode construction comprisinga central heater element and a co-operating equipotential cathodesurface which is positioned immediately adjacent to the heater element.The thermal energy of the heater element may be transferred to thecathode surface. either by conduction or by radiation.

With these and other objects and applicatlons in yiew, our inventionfurther consists in the combinations and details of construction and inthe circuit arrangements hereinl after more fully set forth and claimedand illustrated in the accompanying drawings, wherein:

Figure 1 is a front elevational view of an evacuated electric deviceembodying our invention in a preferred form, a portion of the containingWalls of the envelope being broken away and the grid and plate elementsbeing shown in longitudinal section,

Fig. 2 is a side elevational View of the construction of Fig. 1..

Fig. 3 is an enlarged detailed longitudinal sectional view of thecathode construction of Figs. 1 and 2.

Fig. 4 is a View, similar to Fig. 3, showing a modification in the formof the Amember constituting the equipotential cathode surface and alsoshowing a modification in the means for insulatingly supporting andseparating the branch portions of the heater element.

Fig. 5 is a View, similarto Fig. 4, showing an alternative constructionfor the equipotential cathode surface,

Fig. 6 is a view similar to Fig. 5, but showing the member forming theoperating lcathode surface insulatingly carried by the heater element,

Figs. 7 and 8 are enlarged front and side Adetail elevational views of afurther modified form of cathode construction embodying our invention,

Fig.I4 9 is a side elevational d etail view illustrating our inventionapplied to a.vac num-tube construction emp oymg a palr of controllingelements,

Figs. 10 and 11 are enlarged side elevational and top plan views of acathode construction wherein the equipotential cathode surface isenergized by thermal radiation rather than by thermal conduction, as inthe preceding figures,

Fig. 12 is a diagran'imatic view of circuits and apparatus embodying oneform of our invention and illustrating a circuit arrangement wherein theequipotential cathode element is connected to the heater clement withinthe inclosing envelope,

Fig. 13 is a View similar to Fig. 12 but showing a circuit arrangementwherein an additional lead for the equipotential cathode element isbrought out from the tube permitting the usual vacuum-tube circuits tobe electrically independent of the supply circuits for the heaterelement, and

Fig. 14 is a diagrammatic view of circuits and apparatus employing thevacuum-tube construction shown in F ig. 9."

In a preferred form of embodiment of our invention, as shown in Figs. 1,2 and 3, we provide an elongated envelope 1 having a reentrant portion 2terminating in a supporting press 3. A cathode construction 4 comprisesa mass of refractory material 5 in the form of a slender solid cylinderprovided with a pair of adjacently positioned perforations 6 and 7 whichextend between the ends of the cylinder 5. The perforations 6 and 7 areof such dimensions as to receive a filamentary heating element 8 whichis threaded up through one perforation and down through the other,providing a filament of inverted U-shaped having parallelly extendk ingportions 9 and 11 and a top portion 12.

The distance between the parallel portions 9 and 11 of the filament 8 isso small that the magnetic field established by currents traversing oneportion or section substantially neutralizes the magnetic fieldestablished by currents traversing the other section, thereby avoidingone of the causes for variations in the plate current of vacuum tubeswhen employing alternating currents for the energization of the. cathodeelement. The filament 8 may be energized by connecting the same tofilament supply conductors 13 and 14 which are supported in the press 3and are provided with external extensions 15 and 16, respectively. 1

The cylindrical member 5 is preferably made of some insulatingrefractory material which, when heated to the temperature of the heaterelement 8, is free from chemical action therewith. In the course of muchexperimental work, we have found that Zircon possesses such desirablecharacteristics.

A member 17, which forms the equipotential cathode surface, is shown inthe form of a tube having its inner surface closely embracing thetubular insulating member 5. The outer surface of the member 17 may beCoated with oxides of barium, strontium or other substance which isrendered 'thermionically active when subjected to heat.

The cathode construction 4 may be supported at its upper end by means ofa carrier rod 18 extending from the press 3, as hereafter described. Inpractice, it has been found desirable to provide a spring connectionbetween the cathode construction 4v and the supporting carrier rod 18 inorder to provide for the expansion and contraction of the heatingelement 8. The desired result is obtained by welding a strip 19 to theupper end of the cathode construction, preferably to the tubular member17, and connecting the strip 19 and the carrier rod 18 by means of aspring member 21. The equipotential cathode member 17 may beelectrically connected externally of the tube 1 by Connecting the sameto one of the supply conductors, for

example, conductor 13, by means of a conductor 22.

The cathode construction 4 may be surrounded by plate and grid elements23 and 24 of conventional design. As a matter of illustration, the grid24 is shown in the form of a helical member with its axis coincidingwith the longitudinal axis of the heater element 8. The grid 24 issupported in position by means of a carrier rod 25 extending from thepress 3 and welded to the several helices thereol to provide additionalstili"- ness to the grid construction.

The plate 23 is a tubular member symmetrically positioned with respectto the previlously mentioned elements and supported by carrier rods 26and 27 which are mounted in the press 3. One of the carrier rods, say27, may be extended through the press 3 to form an external. circuitterminal 28.

The envelope 1 may be provided with a base 29, which comprises a solidcircular insulating member 30 and a collar 31, opposite ends of thelatter rigidly embracing the reentrant tube port-ion 2 of the evacuatedelectric device and the insulating member 30, in any approved manner.The outer end of the insulatingy block 30 may be provided .with a raisedportion or base 32 and a plurality of hollowfterminal pins 33, 34, 35and 36. The several external connections are disposed in the hollowportions of the terminal pins such that pins 33 and 34 are connected tothe terminals of the heater element and pins 35 and 36 are connected tothe terminals of the plate and grid elements 23 and 24, respectively.The pin element 33 also serves as a terminal connection for theequipotential vcathode struction 37 which differs from that previ- Iously described in the following important respects. The adjacentsections 9 and 11 of the heater element 8 are insulatingly supported andspaced by means of separate tubular members 38 and 39, respectively. Thetubular members 38 and 39 may be made of any refractory material havingproperties similar to those described for the tubular su porting member5 shown in Fig. 3. T e cathode construction herein shown has anequipontential electron-emitting element in the form ofacylindricalcasing 41 having a cap portion 42 which iselectricallyconnected to the bent portion 12 of the heater filament 8.The structure may besupported b means of a carrier rod 43 which is weldeto the cap portion 42. The carrier rod 43 may be extended through thepress 3 to serve also as an external terminal connection for theequipotential cathode member 41.

In Fig. 5 is shown a cathode construction which is differentiated fromthat of Fig. 4 in the form of the member constituting the equipotentialcathode surface.. In this construction, the cathode surface is formed byspirally Winding an oxide-coated platinum strip 44 around the outerportions of the tubular spacing members 38 and 39.

The construction 45 shown in Fi 6 is distinguishable over the precedingcat ode constructions in that the oxide-coated platinum strip 44 formingthe equipotential cathode surface is wound directly around the sections9 and 11 of the heater element 8, the oxide coating on the platinum stri`44 serving to insulate the stri 44 from t e heating element 8. Ifadditlonal insulation is necessary, the heating element 8 itself may becovered by some insulating oxide, such, for example, as magnesium oxide.The structure ma be supported by means of a carrier rod 46 w ich iswelded to the bent portion 12 of the heater element and to the strip 44.The carrier rod 46 may also serve as a lead for the equipotentialcathode member 44.

VIn Figs. 7 and 8, a cathode construction is shown which differs fromthe foregoin constructions in the following respects. heater element 47,which may Lbe in the form of a flatribbon of tungsten or other suitablematerial, is doubled to form a vpair of adjacently positioned sections48 and 49, as in the preceding figures. The adjacent sections 48 and 49are separated by a thin mica strip 51. Equipotential cathode members 52and 53, in the form of oxide-coated strips of nickel, are positionedimmediately adjacent to the filamentsections 48 and 49, respectively,and are insulatingly spaced therefrom by mica strips 54 and 55. TheAstructure is bound tightly together at the opposite ends thereof bycollar members 56 and 57 and may be supported from the top'by means of acarrier rod 58 `having bail extensions 59 and tial cathode surfacethereof is energized by means of thermal conduction, the insulatingmeans separating the heater element and the equipotential cathode memberserving as the geilt conducting means between the two mem'- ers.

Figs. 10 andll are greatly enlarged views of a construction 64 whereinan equipotential cathode member 65 is heated by thermal radi- ,ationrather than by thermal conduction.

The construction there shown comprises a heater element 66 in the formof adjacently positioned parallelly extending filamentary sections 67and 68, the sections being adjacently positioned to reduce thedistorting effects of the varying magnetic fields estab lished by thefilament exciting currents as. previously described. Adjacent ends ofthefilament sections are welded to a supporting member 69 of nickel orother suitablecon-4 ducting material. The equipotential cathode member65 is shown in the form of a cylindrical casing inclosing the parallellyextending sections of the heater element 66 and is rigidly vsecuredltothe supporting block 69. The construction may be supported by means of acarrier rod 71 having one end rigidly secured to the supporting block69. f

Fig. 12 is a diagrammatic View of the vacuum-tube construction shown inFigs. 1, 2 and 3 together with circuit connections whereby the tube mayfunction as an amplifier of alternating currents. The grid 24 and theequipotential cathode 17 are connected by conductors 72 and 73,respectively, to a source of incoming signals (not shown). Aplatefilament circuit 74, which extends from the plate l23 to theequipotential cathode element 17 ,includes a detecting device 75 and asource 76 of direct-current energy. A source yof al- Aternating-current`energy (not shown) is operatively connected to the heater element 8through a transformer 77. It is noted that, in this arrangement, theequipotential cathode member 17 is connected to the heater element 8within the evacuated portions of the tube 1, thereby requiring only-four terminal pins in the base 2 Thesystem shown in Fig. 13 isdistinguishable over that of Fig. 12 in the provision of separateconnections for the equipotential member 17 and for the heater element8, i

thereby making the energizing circuits for the heater element 8electrically independent of the usual vacuum-tube grid, filament andplate circuits. Such arrangement, however, necessitates the provision ofan additional plug element in the base 29 of the tube as indicated at inFig. 13.

The system shown in Fig. 14 illustrates one application of thevacuum-tube construction (SQ shown in Fig. 9. The additional gridelement 63 is connected to the equipotential cathode member 17 by meansof a conductor 78 including a source `79 of direct-current energytending to malte the grid 63 positive with respect to the equipotentialsurface. When such condition obtains, the grid 63 serves as anelectrostatic screen tending to further decrease the tendency of thealter- -nating exciting currents to vary the tube characteristics. lVhenthe grid 63 is made positive, as shown in the drawing, it forms, ineffect, an artificial cathode, all as will be readily understood bythose skilled in the art.

In practice, we have obtained remarkably high voltage amplification withvacuum tubes employing an indirectly heated cathode surface, asdescribed in the foregoing portlons of the specification. Thelow-resistance type of vacuum tube heretofore employed has a plateimpedance of from 15000 to 25000 ohms, with amplification factorsranging from 5 to 7, whereas the operating characteristics of aVacuum-tube device embodying our invention are such that a tube may bedesigned having a plate impedance of 10000 ohms and a'ivolt` ageamplification factor of 10. Thus, 1t 1s seen that the figure of merit,which is the ratio of the amplification factor squared to the plateresistance, is, in a vacuum-tube construction embodying our invention,approximately four times greater than that of the ordinary tubesheretofore employed.

IVhile we have shown a number of embodiments of our invention, for thepurpose of describing the same and illustrating their principles ofoperation, it is apparent that various changes and modifications may bemade in the nature and the mode of operation and in the details ofconstruction Without departing from the spirit of our invention. IVedesire, therefore, that only such limitations shall be imposed thereonas are indicated by the appended claims or demanded by the prior art.

IVe claim as our invention:

1. Incombination, an equipotential cath- Aede structure comprising anequipotential surface, a substantially non-inductive electrical heaterfor rendering said surface thermionically active and an alternatingcurrent supply circuit operatively associated with said electricalheater for energizing the same.

2. In a cathode structure, a refractory member and a filament comprisingbranch portions disposed in said member, said branch portions beingadjacent one another and the magnetic fields established by currentstraversing the branch portions balancing one another.

3. In combination, a cathode structure comprising an equipotentialcathode member, meansfor rendering said cathode member themionicallyactive and means for resiliently supporting said member and saidfirst-mentioned means.

4. In a vacuum-tube device, a iilamentary heater, a cathode surroundingthe same and heated therefrom, a refractory body electrically insulatingsaid heater from said cathode and a source of alternating currentsupplying said heater, said heater being so constructed that themagnetic field is substintially nil at all points outside said catho e.

5. In a vacuum-tube device, a refractory body, a heat-ing conductorcolnprising two adjacent portions and a cathode surrounding Said twoadjacent portions of said heating conductor and electrically insulatedtherefrom by said refractory body, the distance between said adjacentportions being small relative to the dimensions of said cathode, themagnetic field outside said cathode, due to current in said heatingconductor, being substantially nil.

6. An equipotential cathode comprising a core of refractory materiahasurface of material adapted to emit electrons freely when heatedsupported by said core, and a heatingelement within said core comprising.outgoing and return conductors separated by the minimum distance thatmechanical strength will permit.

7. In a system comprising a device adapted to transform electricalenergy into sound, a source of electromotive force therefor, an electrontube having an anode and a .thermionically-emissive cathodeenergizedfrom an alternating-current source through a heatlng element havingconductors closely adjacent each other the magnetic field of said l?conductors being substantially neutralized between said anode andcathode and poducing no noticeable sound from said device.

8. In combination with an anode, a control electrode, a cathode adaptedto emit electro,ns when heated, a separate heater therefor and a sourceof uctuating current connected to said heater, and a common junctionbetween said cathode and said heater.

9. In a vacuum-tube device, an alternating current filamentary heatercomprising branch portions adjacent one another, a cathode surroundingthe same and heated therefrom, and a refractory body electricallyinsulating said heater from said cathode, the magnetic field from saidheaterbeing substmtialy nil at all points outside said catho e.

10. In combination, a vacuum-tight envelope containing an anode and anequipotential cathode structure comprising an equipoand a source ofconstant'voltage interconnecting said cathode and said anode.

11. In a vacuum-tube device, an anode, a heater, a cathode surroundingthe same and heated therefrom, a refractory body electrically insulatingsaid heater from said cathode, a source of alternating current supplyingsaid heater, said heater being so constructed that the magnetic eldtherefrom is substantially nil at all points outside said cathode, and asource of constant voltage interconnecting said cathode and said anode.

12. In combination with an anode, a control electrode, a cathode adaptedto emit electrons when heated, a separate heater therefor and a sourceof iuctuating current connected to said heater, a common junctionbetween said cathode and said heater, and a source of constant voltageinterconnecting said cathode and said anode.

13. In a vacuum-tube device, a heater, a cathode surrounding the sameand heated therefrom, a refractory body electrically insulating saidheater from said cathode and a source of alternating current supplyingsaid heater, said heater being fs'o constructed that the magnetic fieldtherefrom is substantialy nil at all points outside said cathode.

14. In combination with an anode, a control electrode, a cathode adaptedto emit electrons when heated, a separate heater therefor, a source offluctuating current connected to said heater, and a connectionsubstantially devoid of reactance between said cathode and said heater.

15. In combination with an anode, acontrol electrode, a cathode adaptedto emit electrons when heated,` a separate heater therefor, a source offluctuating current connected to said heater, and a connectionsubstantially devoid of impedance between said cathode and said heater;

16. In combination, an evacuated container, a cathode adapted to emitelectrons freely when heated, an electric heater for said cathode soformed that substantially no magnetic field is vproduced thereby in thespace adjacent the surface of said cathode, and means for electricallyinsulating the major portion of said heater from said cathode.

17. A cathode for a space current device, comprising a conductingmemberhaving a plurality of. sections capable of being heated by anelectric current, said sections being spaced from one another by arefractory material and so disposed with respect to each other that themagnetic fields, produced by the current flowing Athrough said members,

are caused to neutralize each other to prevent magnetic deviation ofspace current fiow.

18. In combination, a cathode comprising a tubular member and aconducting member extending through said tubular member,l a source ofalternating current connected to said conducting member, said membersbeing so constructed and arranged that the magnetic fields produced bythe currents Howing in the members will substantially neutralize eachother.

19. In a three-electrode vacuum-tube, a cathode for emitting a flow ofelectrons therefrom to the plate, and heated by a source of alternatingcurrent, the cathode having a low -voltage drop and a relatively largecrosssectional area and having a heater which is doubled upon itself sothat its opposite leg portions react upon each other to reduce theelectromagnetic effect of the heating current to such a value that thevoltage temperature `and electromagnetic effects on the said fiow ofelectrons to the plate of the cyclic changes in the heating current areneutralized.

20. The combination with a space' discharge vessel'comprising an anode,a control electrode, a cathode and. cathode heating means, a source ofalternating current connected to said heating means, means for supplyinganode potential to said anode, and means for preventing heating currentin said heating means from causing variation in the average voltagebetween the cathode and the anodeduring a half-cycle of`alternatingcurrent. y

21. In a vacuum-tube device, an anode, a grid, a source of periodicheating current of audible frequency and a conductor within the tubeheated thereby, said conductor having closely adjacent parallel portionsconnected in series, said conductor being so connected that theresultant field from said portions is insufficient to affect the spacecurrent substantially.

22. The combination with aspace discharge vessel comprising an anode, acontrol electrode, a cathode and cathode heating means in closeproximity thereto, means supplying alternating current connected to saidheating means, means for supplying anode potential to said anode, andmeans for preventing heating current in said heating means from causingvariation in the average voltage between the cathode and the anodeduring a half-cycle of alternating-current.

23. An electron tube having an electron emitting cathode capable ofbeing heated by alternating current sealed therein and comprising aninner heating element in an outer metallic sheath, coated with alkalineearth oxides, separated from the heating element by refractoryinsulating material, said core `having its opposite end portionsdisposed in closely spaced relation whereby to neutralize the inductiveeffect of the alternating heating current.

In testimom7 whereof, We have hereunto subscribed our names this 30thday of December, 1922.

HUBERT M. FREEMAN. WALLACE G. WADE.

